Amides are excellent mimics of phosphate internucleoside linkages and are well tolerated in short interfering RNAs

Synthesis and Properties of RNA Modified with Thioamide Internucleoside Linkage

Recent success of RNA therapeutics has reinvigorated interest in chemical modifications of RNA. As exemplified by the phosphorothioates, modifications of sugar-phosphate backbone have been remarkably impactful but relatively underexplored in therapeutic RNAs. The present study reports synthesis, thermal stability, and RNA interference activity of RNAs modified with thioamide linkages. Compared to the previously studied amide-modified RNA, thioamide linkages strongly destabilized a short self-complementary RNA model duplex. However, in short interfering RNAs amides and thioamides had a similar effect on duplex stability and target RNA cleavage activity and specificity. Hence, the thioamide may be added to the toolbox of chemical biologist as a useful backbone modification well tolerated by the RNA interference machinery.

Deciphering the role of siRNA in anxiety and depression

Anxiety and depression are central nervous system illnesses that are among the most prevalent medical concerns of the twenty-first century. Patients with this condition and their families bear psychological, financial, and societal hardship. There are currently restrictions when utilizing the conventional course of treatment. RNA interference is expected to become an essential approach in anxiety and depression due to its potent and targeted gene silencing. Silencing of genes by post-transcriptional modification is the mechanism of action of small interfering RNA (siRNA). The suppression of genes linked to disease is typically accomplished by siRNA molecules in an efficient and targeted manner. Unfavourable immune responses, off-target effects, naked siRNA instability, nuclease vulnerability, and the requirement to create an appropriate delivery method are some of the challenges facing the clinical application of siRNA. This review focuses on the use of siRNA in the treatment of anxiety and depression.

Advances in structural-guided modifications of siRNA

To date, the US Food and Drug Administration (FDA) has approved six small interfering RNA (siRNA) drugs: patisiran, givosiran, lumasiran, inclisiran, vutrisiran, and nedosiran, serving as compelling evidence of the promising potential of RNA interference (RNAi) therapeutics. The successful implementation of siRNA therapeutics is improved through a combination of various chemical modifications and diverse delivery approaches. The utilization of chemically modified siRNA at specific sites on either the sense strand (SS) or antisense strand (AS) has the potential to enhance resistance to ribozyme degradation, improve stability and specificity, and prolong the efficacy of drugs. Herein, we provide comprehensive analyses concerning the correlation between chemical modifications and structure-guided siRNA design. Various modifications, such as 2'-modifications, 2',4'-dual modifications, non-canonical sugar modifications, and phosphonate mimics, are crucial for the activity of siRNA. We also emphasize the essential strategies for enhancing overhang stability, improving RISC loading efficacy and strand selection, reducing off-target effects, and discussing the future of targeted delivery.

Synthesis and Properties of RNA Modified with Cationic Amine Internucleoside Linkage

Chemical modifications of RNA are important tools for the development of RNA therapeutics. The present study reports a novel RNA backbone modification that replaces the negatively charged phosphate with a positively charged amine linkage. Despite being thermally destabilizing in RNA duplexes, the amine linkage caused a relatively modest decrease of activity of a modified short interfering RNA (siRNA). At position 2 of the guide strand, the amine modification strongly enhanced the specificity of siRNA while causing an ∼5-fold drop of on-target activity. These results support the future development of amines as cationic RNA modifications and novel tools to modulate protein-RNA interactions.

Synthesis, Duplex-Forming Ability, and Enzymatic Stability of Oligonucleotides Modified with Amide-Linked Dinucleotides Containing a 3',4'-Tetrahydropyran-Bridged Nucleic Acid

Replacement of a phosphodiester linkage with an amide linkage can improve the binding affinity of oligonucleotides to complementary RNA and their stability toward nucleases. In addition, restricting the conformation of the sugar moiety and the phosphate backbone in oligonucleotides effectively improves duplex stability. In this study, we designed amide-linked dinucleotides containing a 3',4'-tetrahydropyran-bridged nucleic acid (3',4'-tpBNA) with a constrained sugar conformation as well as a torsion angle ε. Phosphoramidites of the designed dinucleotides were synthesized and incorporated into oligonucleotides. Conformational analysis of the synthesized dinucleotides showed that the sugar conformation of the S-isomer of the amide-linked dinucleotide containing 3',4'-tpBNA was N-type, which has the same conformation as that of the RNA duplex, while that of another R-isomer was S-type. Tm analysis indicated that the oligonucleotides containing the synthesized S-isomer showed RNA-selective hybridizing ability, although their duplex-forming ability was slightly inferior to that of natural oligonucleotides. Interestingly, the stability of the oligonucleotides toward endonucleases was significantly improved by modification with the two types of amide-linked dinucleotides developed in this study.

Development of nucleic acid medicines based on chemical technology

Oligonucleotide-based therapeutics have attracted attention as an emerging modality that includes the modulation of genes and their binding proteins related to diseases, allowing us to take action on previously undruggable targets. Since the late 2010s, the number of oligonucleotide medicines approved for clinical uses has dramatically increased. Various chemistry-based technologies have been developed to improve the therapeutic properties of oligonucleotides, such as chemical modification, conjugation, and nanoparticle formation, which can increase nuclease resistance, enhance affinity and selectivity to target sites, suppress off-target effects, and improve pharmacokinetic properties. Similar strategies employing modified nucleobases and lipid nanoparticles have been used for developing coronavirus disease 2019 mRNA vaccines. In this review, we provide an overview of the development of chemistry-based technologies aimed at using nucleic acids for developing therapeutics over the past several decades, with a specific emphasis on the structural design and functionality of chemical modification strategies.

Syntheses of Pyrimidine-Modified Seleno-DNAs as Stable Antisense Molecules

Chemically modified antisense oligonucleotides (ASO) currently in pre-clinical and clinical experiments mainly focus on the 2'-position derivatizations to enhance stability and targeting affinity. Considering the possible incompatibility of 2'-modifications with RNase H stimulation and activity, we have hypothesized that the atom specific modifications on nucleobases can retain the complex structure and RNase H activity, while enhancing ASO's binding affinity, specificity, and stability against nucleases. Herein we report a novel strategy to explore our hypothesis by synthesizing the deoxynucleoside phosphoramidite building block with the seleno-modification at 5-position of thymidine, as well as its Se-oligonucleotides. Via X-ray crystal structural study, we found that the Se-modification was located in the major groove of nucleic acid duplex and didn't cause the thermal and structural perturbations. Surprisingly, our nucleobase-modified Se-DNAs were exceptionally resistant to nuclease digestion, while compatible with RNase H activity. This affords a novel avenue for potential antisense modification in the form of Se-antisense oligonucleotides (Se-ASO).

Amide Modifications in the Seed Region of the Guide Strand Improve the On-Target Specificity of Short Interfering RNA

RNA interference (RNAi) is a well-established research tool and is also maturing as a novel therapeutic approach. For the latter, microRNA-like off-target activity of short interfering RNAs (siRNAs) remains as one of the main problems limiting RNAi drug development. In this communication, we report that replacement of a single internucleoside phosphodiester in the seed region (nucleotides 2 to 7) of the guide strand with an amide linkage suppressed the undesired microRNA-like off-target activity by at least an order of magnitude. For the specific siRNA targeting the PIK3CB gene, an amide modification between the third and fourth nucleotides of the guide strand showed the strongest enhancement of specificity (completely eliminated off-target silencing) while maintaining high on-target activity. These results are important because off-target activity is one of the main remaining roadblocks for RNA based drug development.

Optimization of Automated Synthesis of Amide-Linked RNA

The recent FDA approval of several antisense and siRNA drugs illustrates the utility of nucleic acid chemical modifications, but numerous challenges remain for generalized nucleic acid therapeutics, urging the exploration of new modification strategies. Replacing backbone phosphates with amides has shown promise for enhancing siRNA activity, specificity, and nuclease resistance; however, amide-linked RNA has not been fully explored due to lengthy and low yielding manual amide coupling procedures. We have addressed this by automating the assembly of amide-linked RNA using an Expedite 8909 nucleic acid synthesizer and optimizing solid-phase synthesis conditions to achieve 91-95% yields in just 5 min of coupling time. The optimized protocol allowed synthesis of a 21-nucleotide-long siRNA guide strand having six consecutive amide linkages at the 3'-end with an overall yield of ∼1%. Our results show that the steric hindrance caused by bulky 2'-O protecting groups and steric hindrance of the solid support are the key optimization variables for improving the amide couplings.

SiRNAs with Neutral Phosphate Triester Hydrocarbon Tails Exhibit Carrier-Free Gene-Silencing Activity

Short interfering RNAs (siRNAs) show promise as gene-silencing therapeutics, but their cellular uptake remains a challenge. We have recently shown the synthesis of siRNAs bearing a single neutral phenylethyl phosphotriester linkage within the sense strand. Here, we report the synthesis of siRNAs bearing three different hydrophobic phosphate triester linkages at key positions within the sense strand and assess their gene silencing in the absence of a transfection carrier. The best siRNAs bearing hydrophobic phosphate triester tails were not aromatic and exhibited effective gene silencing (IC₅₀ ≈ 56-141 nM), whereas the aromatic derivative with three hydrophobic tails did not exhibit carrier-free gene silencing.

Amide Internucleoside Linkages Are Well Tolerated in Protospacer Adjacent Motif-Distal Region of CRISPR RNAs

The development of CRISPR-Cas9 mediated gene editing technology is revolutionizing molecular biology, biotechnology, and medicine. However, as with other nucleic acid technologies, CRISPR would greatly benefit from chemical modifications that optimize delivery, activity, and specificity of gene editing. Amide modifications at certain positions of short interfering RNAs have been previously shown to improve their RNAi activity and specificity, which motivated the current study on replacement of selected internucleoside phosphates of CRISPR RNAs with amide linkages. Herein, we show that amide modifications did not interfere with CRISPR-Cas9 activity when placed in the protospacer adjacent motif (PAM) distal region of CRISPR RNAs. In contrast, modification of the seed region led to a loss of DNA cleavage activity at most but not all positions. These results are encouraging for future studies on amides as backbone modifications in CRISPR RNAs.

Structure and hybridization properties of phosphoryl guanidine oligonucleotides under crowding conditions

Phosphoryl guanidine oligonucleotides (PGOs) are promising uncharged analogs of nucleic acids and are used in a variety of applications. The importance of hydration is frequently ignored during the design of modified nucleic acid probes. Such hydrophobic modifications (phosphoryl guanidine) are expected to have a significant impact on the structure and thermal stability of the affected oligo with complementary nucleic acids. Here we aimed to investigate (by the osmotic stress method) hydration changes upon the formation of a duplex of a PGO with complementary DNA. According to our results, the presence of phosphoryl guanidines in one or both strands of a duplex only minimally affects hydration alterations under crowding conditions. The secondary structure of native and modified duplexes did not change significantly in the presence of ethanol, ethylene glycol, polyethylene glycol 200, or polyethylene glycol 1000. After the addition of a cosolvent, the thermodynamic stability of the PGO complexes changed in the same manner as that seen in a corresponding DNA duplex. The findings reported here and our previous studies form the basis for efficient use of PGOs in basic research and a variety of applications.

Building siRNAs with Cubes: Synthesis and Evaluation of Cubane-Modified siRNAs

Cubane molecules hold great potential for medicinal chemistry applications due to their inherent stability and low toxicity. In this study, we report the synthesis of a cubane derivative phosphoramidite for the incorporation of cubane into small interfering RNAs (siRNAs). Synthetic siRNAs rely on chemical modifications to improve their pharmacokinetic profiles. However, they are still able to mediate sequence-specific gene silencing via the endogenous RNA interference pathway. We designed a library of siRNAs bearing cubane at different positions within the sense and antisense strands. All siRNAs showed excellent gene-silencing activity, with IC₅₀ values ranging from 45.4 to 305 pM. Incorporating the cubane modification in both the sense and antisense strand led to viable duplexes with good biological activity. To the best of our knowledge, this is the first report of siRNAs bearing a cubane derivative within the backbone.

Beyond ribose and phosphate: Selected nucleic acid modifications for structure-function investigations and therapeutic applications

Over the past 25 years, the acceleration of achievements in the development of oligonucleotide-based therapeutics has resulted in numerous new drugs making it to the market for the treatment of various diseases. Oligonucleotides with alterations to their scaffold, prepared with modified nucleosides and solid-phase synthesis, have yielded molecules with interesting biophysical properties that bind to their targets and are tolerated by the cellular machinery to elicit a therapeutic outcome. Structural techniques, such as crystallography, have provided insights to rationalize numerous properties including binding affinity, nuclease stability, and trends observed in the gene silencing. In this review, we discuss the chemistry, biophysical, and structural properties of a number of chemically modified oligonucleotides that have been explored for gene silencing.

Modified internucleoside linkages for nuclease-resistant oligonucleotides

In the past few years, several drugs derived from nucleic acids have been approved for commercialization and many more are in clinical trials. The sensitivity of these molecules to nuclease digestion in vivo implies the need to exploit resistant non-natural nucleotides. Among all the possible modifications, the one concerning the internucleoside linkage is of particular interest. Indeed minor changes to the natural phosphodiester may result in major modifications of the physico-chemical properties of nucleic acids. As this linkage is a key element of nucleic acids' chemical structures, its alteration can strongly modulate the plasma stability, binding properties, solubility, cell penetration and ultimately biological activity of nucleic acids. Over the past few decades, many research groups have provided knowledge about non-natural internucleoside linkage properties and participated in building biologically active nucleic acid derivatives. The recent renewing interest in nucleic acids as drugs, demonstrated by the emergence of new antisense, siRNA, aptamer and cyclic dinucleotide molecules, justifies the review of all these studies in order to provide new perspectives in this field. Thus, in this review we aim at providing the reader insights into modified internucleoside linkages that have been described over the years whose impact on annealing properties and resistance to nucleases have been evaluated in order to assess their potential for biological applications. The syntheses of modified nucleotides as well as the protocols developed for their incorporation within oligonucleotides are described. Given the intended biological applications, the modifications described in the literature that have not been tested for their resistance to nucleases are not reported.

Towards Zwitterionic Oligonucleotides with Improved Properties: the NAA/LNA-Gapmer Approach

Oligonucleotides (ON) are promising therapeutic candidates, for instance by blocking endogenous mRNA (antisense mechanism). However, ON usually require structural modifications of the native nucleic acid backbone to ensure satisfying pharmacokinetic properties. One such strategy to design novel antisense oligonucleotides is to replace native phosphate diester units by positively charged artificial linkages, thus leading to (partially) zwitterionic backbone structures. Herein, we report a "gapmer" architecture comprised of one zwitterionic central segment ("gap") containing nucleosyl amino acid (NAA) modifications and two outer segments of locked nucleic acid (LNA). This NAA/LNA-gapmer approach furnished a partially zwitterionic ON with optimised properties: i) the formation of stable ON-RNA duplexes with base-pairing fidelity and superior target selectivity at 37 °C; and ii) excellent stability in complex biological media. Overall, the NAA/LNA-gapmer approach is thus established as a strategy to design partially zwitterionic ON for the future development of novel antisense agents.

Amide-Modified RNA: Using Protein Backbone to Modulate Function of Short Interfering RNAs

RNA-based technologies to control gene expression, such as RNA interference (RNAi) and CRISPR-Cas9, have become powerful tools in molecular biology and genomics. The exciting potential that RNAi and CRISPR-Cas9 may also become new therapeutic approaches has reinvigorated interest in chemically modifying RNA to improve its properties for in vivo applications. Chemical modifications can improve enzymatic stability, in vivo delivery, cellular uptake, and sequence specificity as well as minimize off-target activity of short interfering RNAs (siRNAs) and CRISPR associated RNAs. While numerous good solutions for improving stability toward enzymatic degradation have emerged, optimization of the latter functional properties remains challenging. In this Account, we discuss synthesis, structure, and biological activity of novel nonionic analogues of RNA that have the phosphodiester backbone replaced by amide linkages (AM1). Our long-term goal is to use the amide backbone to improve the stability and specificity of siRNAs and other functional RNAs. Our work in this area was motivated by early discoveries that nonionic backbone modifications, including AM1, did not disturb the overall structure or thermal stability of RNA duplexes. We hypothesized that the reduced negative charge and hydrophobic nature of the AM1 backbone modification might be useful in optimizing functional applications through enhanced cellular uptake, and might suppress unwanted off-target effects of siRNAs. NMR and X-ray crystallography studies showed that AM1 was an excellent mimic of phosphodiester linkages in RNA. The local conformational changes caused by the amide linkages were easily accommodated by small adjustments in RNA's conformation. Further, the amide carbonyl group assumed an orientation that is similar to one of the nonbridging P-O bonds, which may enable amide/phosphate mimicry by conserving hydrogen bonding interactions. The crystal structure of a short amide-modified DNA-RNA hybrid in complex with RNase H indicated that the amide N-H could also act as an H-bond donor to stabilize RNA-protein interactions, which is an interaction mode not available to phosphate groups. Functional assays established that amides were well tolerated at internal positions in both strands of siRNAs. Surprisingly, amide modifications in the middle of the guide strand and at the 5'-end of the passenger strand increased RNAi activity compared to unmodified siRNA. Most importantly, an amide linkage between the first and second nucleosides of the passenger strand completely abolished its undesired off-target activity while enhancing the desired RNAi activity. These results suggest that RNAi may tolerate more substantial modifications of siRNAs than the chemistries tried so far. The findings are also important and timely because they demonstrate that amide modifications may reduce off-target activity of siRNAs, which remains an important roadblock for clinical use of RNAi. Taken together, our work suggests that amide linkages have underappreciated potential to optimize the biological and pharmacological properties of RNA. Expanded use of amide linkages in RNA to enhance CRISPR and other technologies requiring chemically stable, functional mimics of noncoding RNAs is expected.

Synthesis and Evaluation of Neutral Phosphate Triester Backbone-Modified siRNAs

Two unsymmetrical dinucleotide phosphate triesters were synthesized via transesterification from tris(2,2,2-trifluoroethyl) phosphate. The protected triesters were phosphytilated to generate phosphoramidites for solid-phase oligonucleotide synthesis. Neutral phenylethyl phosphate-modified short-interfering RNAs (siRNAs) were synthesized and evaluated for their gene-silencing ability, siRNA strand selection, and resistance to nucleases. These backbone-modified phosphate triester siRNAs offer many improvements compared to natural unmodified siRNAs.

Synthesis and Biological Activity of Short Interfering RNAs Having Several Consecutive Amide Internucleoside Linkages

The success of RNA interference (RNAi) as a research tool and potential therapeutic approach has reinvigorated interest in chemical modifications of RNA. Replacement of the negatively charged phosphates with neutral amides may be expected to improve bioavailability and cellular uptake of small interfering RNAs (siRNAs) critical for in vivo applications. In this study, we introduced up to seven consecutive amide linkages at the 3'-end of the guide strand of an siRNA duplex. Modified guide strands having four consecutive amide linkages retained high RNAi activity when paired with a passenger strand having one amide modification between its first and second nucleosides at the 5'-end. Further increase in the number of modifications decreased the RNAi activity; however, siRNAs with six and seven amide linkages still showed useful target silencing. While an siRNA duplex having nine amide linkages retained some silencing activity, the partial reduction of the negative charge did not enable passive uptake in HeLa cells. Our results suggest that further chemical modifications, in addition to amide linkages, are needed to enable cellular uptake of siRNAs in the absence of transfection agents.

Dynamic DNA nanostructures in biomedicine: Beauty, utility and limits

DNA composite materials are at the forefront, especially for biomedical science, as they can increase the efficacy and safety of current therapies and drug delivery systems. The specificity and predictability of the Watson-Crick base pairing make DNA an excellent building material for the production of programmable and multifunctional objects. In addition, the principle of nucleic acid hybridization can be applied to realize mobile nanostructures, such as those reflected in DNA walkers that sort and collect cargo on DNA tracks, DNA robots performing tasks within living cells and/or DNA tweezers as ultra-sensitive biosensors. In this review, we present the diversity of dynamic DNA nanostructures functionalized with different biomolecules/functional units, imaging smart biomaterials capable of sensing, interacting, delivery and performing complex tasks within living cells/organisms.

Synthesis and biophysical properties of carbamate-locked nucleic acid (LNA) oligonucleotides with potential antisense applications

Antisense oligonucleotides (ASOs) are becoming important drugs for hard to treat diseases. Modifications to their DNA backbones are essential to inhibit degradation in vivo, but they can reduce binding affinity to RNA targets. To address this problem we have combined the enzymatic resistance of carbamate (CBM) DNA backbone analogues with the thermodynamic stability conferred by locked nucleic acid sugars (LNA). Using a dinucleotide phosphoramidite strategy and automated solid phase synthesis, we have synthesised a set of oligonucleotides modified with multiple LNA-CBM units. The LNA sugars restore binding affinity to RNA targets, and in this respect LNA position with respect to the CBM linkage is important. Oligonucleotides containing carbamate flanked on its 5'and 3'-sides by LNA form stable duplexes with RNA and unstable duplexes with DNA, which is desirable for antisense applications. Carbamate-LNA modified oligonucleotides also show increased stability in the presence of snake venom and foetal bovine serum compared to LNA or CBM backbones alone.

Re-Engineering RNA Molecules into Therapeutic Agents

Efforts to chemically modify nucleic acids got underway merely a decade after the discovery of the DNA double helix and initially targeted nucleosides and nucleotides. The origins of three analogues that remain staples of modification strategies and figure prominently in FDA-approved nucleic acid therapeutics can be traced to the 1960s: 2'-deoxy-2'-fluoro-RNA (2'-F RNA), 2'- O-methyl-RNA (2'- OMe RNA), and the phosphorothioates (PS-DNA/RNA). Progress in nucleoside phosphoramidite-based solid phase oligonucleotide synthesis has gone hand in hand with the creation of second-generation (e.g., 2'- O-(2-methoxyethyl)-RNA, MOE-RNA) and third-generation (e.g., bicyclic nucleic acids, BNAs) analogues, giving rise to an expanding universe of modified nucleic acids. Thus, beyond site-specifically altered DNAs and RNAs with a modified base, sugar, and/or phosphate backbone moieties, nucleic acid chemists have created a host of conjugated oligonucleotides and artificial genetic polymers (XNAs). The search for oligonucleotides with therapeutic efficacy constitutes a significant driving force for these investigations. However, nanotechnology, diagnostics, synthetic biology and genetics, nucleic acid etiology, and basic research directed at the properties of native and artificial pairing systems have all stimulated the design of ever more diverse modifications. Modification of nucleic acids can affect pairing and chemical stability, conformation and interactions with a flurry of proteins and enzymes that play important roles in uptake, transport or processing of targets. Enhancement of metabolic stability is a central concern in the design of antisense, siRNA and aptamer oligonucleotides for therapeutic applications. In the antisense approach, uniformly modified oligonucleotides or so-called gapmers are used to target a specific RNA. The former may sterically block transcription or direct alternative splicing, whereas the latter feature a central PS window that elicits RNase H-mediated cleavage of the target. The key enzyme in RNA interference (RNAi) is Argonaute 2 (Ago2), a dynamic multidomain enzyme that binds multiple regions of the guide (antisense) and passenger (sense) siRNAs. The complexity of the individual interactions between Ago2 and the siRNA duplex provides significant challenges for chemical modification. Therefore, a uniform (the same modification throughout, e.g., antisense) or nearly uniform (e.g., aptamer) modification strategy is less useful in the pursuit of siRNA therapeutic leads. Instead, unique structural features and protein interactions of 5'-end (guide/Ago2MID domain), seed region, central region (cleavage site/Ago2 PIWI domain), and 3'-terminal nucleotides (guide/Ago2 PAZ domain) demand a more nuanced approach in the design of chemically modified siRNAs for therapeutic use. This Account summarizes current siRNA modification strategies with an emphasis on the regio-specific interactions between oligonucleotide and Ago2 and how these affect the choice of modification and optimization of siRNA efficacy. In addition to standard assays applied to measure the effects of modification on the stability of pairing and resistance against nuclease degradation, structural insights based on crystallographic data for modified RNAs alone and in complex with Ago2 from molecular modeling studies are a valuable guide in the design of siRNA therapeutics. Thus, this comprehensive approach is expected to result in accelerated generation of new siRNA-based therapies against various diseases, now that the first siRNA has obtained approval by the US FDA for treatment of hereditary hATTR amyloidosis.

Studies on sugar puckering and glycosidic stabilities of 3'-amino-5'-carboxymethyl-3',5'-dideoxy nucleoside mimics

The synthesis of nucleoside amino acid monomers and dimers has been carried out to evaluate and characterize the impact of the neutral amide backbone on key attributes like puckering of the sugar rings and glycosidic bond strengths of these analogs. The conformational analysis suggests that amide-linked nucleotides have a high predilection towards N-type conformers. The glycosidic bond strength was found to be slightly weaker compared to ribonucleosides under acidic conditions at high temperatures. The results will be helpful to explore in future the development of fully amide-linked oligonucleotides for therapeutic purposes.

Enhanced Stability of DNA Oligonucleotides with Partially Zwitterionic Backbone Structures in Biological Media

Deficient stability towards nuclease-mediated degradation is one of the most relevant tasks in the development of oligonucleotide-derived biomedical agents. This hurdle can be overcome through modifications to the native oligonucleotide backbone structure, with the goal of simultaneously retaining the unique hybridization properties of nucleic acids. The nucleosyl amino acid (NAA)-modification is a recently introduced artificial cationic backbone linkage. Partially zwitterionic NAA-modified oligonucleotides had previously shown hybridization with DNA strands with retained base-pairing fidelity. In this study, we report the significantly enhanced stability of NAA-modified oligonucleotides towards 3′- and 5′-exonuclease-mediated degradation as well as in complex biological media such as human plasma and whole cell lysate. This demonstrates the potential versatility of the NAA-motif as a backbone modification for the development of biomedically active oligonucleotide analogues.

Oligonucleotide analogues with cationic backbone linkages

Their unique ability to selectively bind specific nucleic acid sequences makes oligonucleotides promising bioactive agents. However, modifications of the nucleic acid structure are an essential prerequisite for their application in vivo or even in cellulo. The oligoanionic backbone structure of oligonucleotides mainly hampers their ability to penetrate biological barriers such as cellular membranes. Hence, particular attention has been given to structural modifications of oligonucleotides which reduce their overall number of negative charges. One such approach is the site-specific replacement of the negatively charged phosphate diester linkage with alternative structural motifs which are positively charged at physiological pH, thus resulting in zwitterionic or even oligocationic backbone structures. This review provides a general overview of this concept and summarizes research on four according artificial backbone linkages: aminoalkylated phosphoramidates (and related systems), guanidinium groups, S-methylthiourea motifs, and nucleosyl amino acid (NAA)-derived modifications. The synthesis and properties of the corresponding oligonucleotide analogues are described.

A Single Amide Linkage in the Passenger Strand Suppresses Its Activity and Enhances Guide Strand Targeting of siRNAs

Potential in vivo applications of RNA interference (RNAi) require suppression of various off-target activities. Herein, we report that replacement of a single phosphate linkage between the first and second nucleosides of the passenger strand with an amide linkage almost completely abolished its undesired activity and restored the desired activity of guide strands that had been compromised by unfavorable amide modifications. Molecular modeling suggested that the observed effect was most likely due to suppressed loading of the amide-modified strand into Ago2 caused by inability of amide to adopt the conformation required for the backbone twist that docks the first nucleotide of the guide strand in the MID domain of Ago2. Eliminating off-target activity of the passenger strand will be important for improving therapeutic potential of RNAi.

Adenosine analogs bearing phosphate isosteres as human MDO1 ligands

The human O-acetyl-ADP-ribose deacetylase MDO1 is a mono-ADP-ribosylhydrolase involved in the reversal of post-translational modifications. Until now MDO1 has been poorly characterized, partly since no ligand is known besides adenosine nucleotides. Here, we synthesized thirteen compounds retaining the adenosine moiety and bearing bioisosteric replacements of the phosphate at the ribose 5'-oxygen. These compounds are composed of either a squaryldiamide or an amide group as the bioisosteric replacement and/or as a linker. To these groups a variety of substituents were attached such as phenyl, benzyl, pyridyl, carboxyl, hydroxy and tetrazolyl. Biochemical evaluation showed that two compounds, one from both series, inhibited ADP-ribosyl hydrolysis mediated by MDO1 in high concentrations.

Oligonucleotides with Cationic Backbone and Their Hybridization with DNA: Interplay of Base Pairing and Electrostatic Attraction

Non‐natural oligonucleotides represent important (bio)chemical tools and potential therapeutic agents. Backbone modifications altering hybridization properties and biostability can provide useful analogues. Here, we employ an artificial nucleosyl amino acid (NAA) motif for the synthesis of oligonucleotides containing a backbone decorated with primary amines. An oligo‐T sequence of this cationic DNA analogue shows significantly increased affinity for complementary DNA. Notably, hybridization with DNA is still governed by Watson–Crick base pairing. However, single base pair mismatches are tolerated and some degree of sequence‐independent interactions between the cationic NAA backbone and fully mismatched DNA are observed. These findings demonstrate that a high density of positive charges directly connected to the oligonucleotide backbone can affect Watson–Crick base pairing. This provides a paradigm for the design of therapeutic oligonucleotides with altered backbone charge patterns.

Amide linkages mimic phosphates in RNA interactions with proteins and are well tolerated in the guide strand of short interfering RNAs

While the use of RNA interference (RNAi) in molecular biology and functional genomics is a well-established technology, in vivo applications of synthetic short interfering RNAs (siRNAs) require chemical modifications. We recently found that amides as non-ionic replacements for phosphodiesters may be useful modifications for optimization of siRNAs. Herein, we report a comprehensive study of systematic replacement of a single phosphate with an amide linkage throughout the guide strand of siRNAs. The results show that amides are surprisingly well tolerated in the seed and central regions of the guide strand and increase the silencing activity when placed between nucleosides 10 and 12, at the catalytic site of Argonaute. A potential explanation is provided by the first crystal structure of an amide-modified RNA-DNA with Bacillus halodurans RNase H1. The structure reveals how small changes in both RNA and protein conformation allow the amide to establish hydrogen bonding interactions with the protein. Molecular dynamics simulations suggest that these alternative binding modes may compensate for interactions lost due to the absence of a phosphodiester moiety. Our results suggest that an amide can mimic important hydrogen bonding interactions with proteins required for RNAi activity and may be a promising modification for optimization of biological properties of siRNAs.

Concurrent Hydrogenation of Three Functional Groups Enables Synthesis of C3'-Homologated Nucleoside Amino Acids

Internucleoside amide linkages are excellent mimics of phosphodiesters in RNA and may be used to optimize the properties of short interfering RNAs. Herein we report a remarkably straightforward, efficient and step economic synthesis of C3'-homologated uridine and adenosine amino acids starting from nucleosides in six steps (31% overall yield) and eight steps (16% overall yield), respectively. The key enabling step is a one-pot multifunctional group transformation including a stereoselective hydrogenation, termed "Global Hydrogenation".

Therapeutic potential of chemically modified siRNA: Recent trends

Small interfering RNAs (siRNAs) are one of the valuable tools to investigate the functions of genes and are also used for gene silencing. It has a wide scope in drug discovery through in vivo target validation. siRNA therapeutics are not optimal drug-like molecules due to poor bioavailability and immunogenic and off-target effects. To overcome the challenges associated with siRNA therapeutics, identification of appropriate chemical modifications that improves the stability, specificity and potency of siRNA is essential. This review focuses on the various chemical modifications and their implications in siRNA therapy.

Effective gene-silencing of siRNAs that contain functionalized spacer linkages within the central region

Short-interfering RNAs containing a variety of functional groups at the central region of the sense strand were synthesized and evaluated.

The Medicinal Chemistry of Therapeutic Oligonucleotides

Oligonucleotide-based therapeutics have made rapid progress in the clinic for treatment of a variety of disease indications. Unmodified oligonucleotides are polyanionic macromolecules with poor drug-like properties. Over the past two decades, medicinal chemists have identified a number of chemical modification and conjugation strategies which can improve the nuclease stability, RNA-binding affinity, and pharmacokinetic properties of oligonucleotides for therapeutic applications. In this perspective, we present a summary of the most commonly used nucleobase, sugar and backbone modification, and conjugation strategies used in oligonucleotide medicinal chemistry.

Structural Basis of Duplex Thermodynamic Stability and Enhanced Nuclease Resistance of 5'-C-Methyl Pyrimidine-Modified Oligonucleotides

Although judicious use of chemical modifications has contributed to the success of nucleic acid therapeutics, poor systemic stability remains a major hurdle. The introduction of functional groups around the phosphate backbone can enhance the nuclease resistance of oligonucleotides (ONs). Here, we report the synthesis of enantiomerically pure (R)- and (S)-5'-C-methyl (C5'-Me) substituted nucleosides and their incorporation into ONs. These modifications generally resulted in a decrease in thermal stability of oligonucleotide (ON) duplexes in a manner dependent on the stereoconfiguration at C5' with greater destabilization characteristic of (R)-epimers. Enhanced stability against snake venom phosphodiesterase resulted from modification of the 3'-end of an ON with either (R)- or (S)-C5'-Me nucleotides. The (S)-isomers with different 2'-substituents provided greater resistance against 3'-exonucleases than the corresponding (R)-isomers. Crystal structure analyses of RNA octamers with (R)- or (S)-5'-C-methyl-2'-deoxy-2'-fluorouridine [(R)- or (S)-C5'-Me-2'-FU, respectively] revealed that the stereochemical orientation of the C5'-Me and the steric effects that emanate from the alkyl substitution are the dominant determinants of thermal stability and are likely molecular origins of resistance against nucleases. X-ray and NMR structural analyses showed that the (S)-C5'-Me epimers are spatially and structurally more similar to their natural 5' nonmethylated counterparts than the corresponding (R)-epimers.

Postsynthetic conjugation of RNA to carboxylate and dicarboxylate molecules

Carboxylates and dicarboxylates are important phosphate mimics. Herein, we present a simple synthetic route for the preparation of RNA carboxylate/dicarboxylate conjugates, starting from suitably protected NH2- and COOH-containing molecules that are coupled to the RNA on the solid support. The key point in our method was the use of trimethylsilylethanol (TMSE-OH) protecting group, which is removed simultaneously with the silyl protecting group on the 2'-OH of the RNA ribose (e.g. t-Butyldimethylsilyl) during the final RNA cleavage/deprotection steps. The usefulness of this method was demonstrated by preparing different RNA-phosphate mimics oligos.

Chimeric RNA Oligonucleotides with Triazole and Phosphate Linkages: Synthesis and RNA Interference

Chimeric RNA oligonucleotides with an artificial triazole linker were synthesized using solution-phase click chemistry and solid-phase automated synthesis. Scalable synthesis methods for jointing units for the chimeric structure have been developed, and after click-coupling of the jointing units with triazole linkers, a series of chimeric oligonucleotides was prepared by utilizing the well-established phosphoramidite method for the elongation. The series of chimeric 21-mer oligonucleotides that possessed the triazole linker at different strands and positions allowed for a screening study of the RNA interference to clarify the preference of the triazole modifications in small-interfering RNA molecules.

Glycine-Linked Nucleoside-β-Amino Acids: Polyamide Analogues of Nucleic Acids

3'-5'-Deoxyribose-sugar-phoshate backbone in DNA is completely replaced by 2'-deoxyribonucleoside-based β-amino acids interlinked by glycine to create uncharged polyamide DNA with 3'-5'-directionality. These oligomers as conjugates of α-amino acids and nucleoside-β-amino acids bind strongly and sequence-specifically only to the antiparallel complementary RNA and DNA.

A semi-supervised tensor regression model for siRNA efficacy prediction

BACKGROUND

Short interfering RNAs (siRNAs) can knockdown target genes and thus have an immense impact on biology and pharmacy research. The key question of which siRNAs have high knockdown ability in siRNA research remains challenging as current known results are still far from expectation.

RESULTS

This work aims to develop a generic framework to enhance siRNA knockdown efficacy prediction. The key idea is first to enrich siRNA sequences by incorporating them with rules found for designing effective siRNAs and representing them as enriched matrices, then to employ the bilinear tensor regression to predict knockdown efficacy of those matrices. Experiments show that the proposed method achieves better results than existing models in most cases.

CONCLUSIONS

Our model not only provides a suitable siRNA representation but also can predict siRNA efficacy more accurate and stable than most of state-of-the-art models. Source codes are freely available on the web at: http://www.jaist.ac.jp/\~bao/BiLTR/ .

Knockdown of STAT3 expression in SKOV3 cells by biodegradable siRNA-PLGA/CSO conjugate micelles

Biodegradable and biocompatible poly(d,l-lactic-co-glycolic acid) (PLGA)was conjugated to the 5'-thiol end of signal transducer and activator of transcription 3 (STAT3) small interfering RNA (STAT3-siRNA) via a disulfide bond. In aqueous environments, these siRNA-PLGA conjugates can spontaneously form core/shell type spherical micelles with a particle size of about 200 nm. A biodegradable, low molecular weight cationic polymer, chitosan oligosaccharide (CSO), was added to the siRNA-PLGA micelles at different nitrogen to phosphate (N/P) ratios to form stable, spherical siRNA-PLGA/CSO micelles with sizes of 150-180 nm. The siRNA-PLGA/CSO micelles were produced via ionic complexation between negatively charged siRNA and positively charged CSO on the outer shell of the micelles. The siRNA-PLGA/CSO micelles exhibited superior cellular uptake and STAT3 gene silencing efficiency in SKOV3 ovarian cancer cells when compared with siRNA/CSO complexes at the same N/P ratios with no significant differences with lipofectamine 2000. Furthermore, the siRNA-PLGA/CSO micelles showed that the efficiencies of cellular uptake and STAT3 gene silencing gradually increased with increasing N/P ratios. The siRNA-PLGA/CSO micelles also inhibited the growth of SKOV3 cells, as well as, promoted apoptosis of the cells. These results indicate that siRNA-PLGA/CSO micelles can be utilized as a novel and efficient siRNA carrier to treat a variety of diseases.

NAA-modified DNA oligonucleotides with zwitterionic backbones: stereoselective synthesis of A-T phosphoramidite building blocks

Modifications of the nucleic acid backbone are essential for the development of oligonucleotide-derived bioactive agents. The NAA-modification represents a novel artificial internucleotide linkage which enables the site-specific introduction of positive charges into the otherwise polyanionic backbone of DNA oligonucleotides. Following initial studies with the introduction of the NAA-linkage at T-T sites, it is now envisioned to prepare NAA-modified oligonucleotides bearing the modification at X-T motifs (X = A, C, G). We have therefore developed the efficient and stereoselective synthesis of NAA-linked 'dimeric' A-T phosphoramidite building blocks for automated DNA synthesis. Both the (S)- and the (R)-configured NAA-motifs were constructed with high diastereoselectivities to furnish two different phosphoramidite reagents, which were employed for the solid phase-supported automated synthesis of two NAA-modified DNA oligonucleotides. This represents a significant step to further establish the NAA-linkage as a useful addition to the existing 'toolbox' of backbone modifications for the design of bioactive oligonucleotide analogues.